Genetic background is known to influence all aspects of mammary biology from normal development and lactation to breast cancer. However, host genetics only explains a fraction of this variation and environmental factors, including those that could be inherited through maternal transmission, still play a major role. High-throughput DNA sequencing technologies have led to an emerging concept that the microbiome is a major contributor to normal development and health. In the mammary gland, there is now known to be both a tissue microbiome as well as a milk microbiome. There is also evidence to support the idea that the intestinal microbiome can influence estrogen dynamics through the activity of the ?estrobolome?, a system of microbial enzymes capable of converting conjugated estrogens secreted in bile to free estrogen, which can be reabsorbed to elevate circulating concentrations. In prepubertal females this elevation would be expected to influence both the onset of puberty and the development of the mammary gland. Recent work has shown that the intestinal microbiome can be directly manipulated through the use of fecal microbiome transplantation (FMT), which can profoundly affect not only intestinal health, but also whole body growth and metabolism. In addition, FMT studies in mice suggest that the microbiome regulates the insulin-like growth factor system, a system that is linked to the regulation of sexual maturation, and that our own lab has shown to regulate mammary ductal development and lactation. Recent 16s pilot data from our team, that was collected in collaboration with Drs. Nadim Ajami and Joseph Petrosino of the Alkek Center of Metagenomics and Microbiome Research, has demonstrated that select inbred mouse strains with differences in mammary development and lactation capacity have striking differences in their intestinal microbiomes. These observations support the hypothesis that the intestinal and mammary microbiomes are dominant mediators of the effects of environment and genetic background on mammary development and lactation, and that these effects occur through modulation of estrogen, progesterone, and/or IGF-I. We will test this hypothesis first by establishing the range of variability in the intestinal, mammary, and milk microbiomes from inbred mouse strains of varying genetic background, and second by measuring the direct impact of microbiome diversity on mammary development and lactation through FMT into gnotobiotic female mice. Measurement of blood and urinary hormone concentrations will track the developmental effects of FMT in relationship to estrogen, progesterone, and IGF-I. The specific aims will be: 1) Establish the diversity and covariance among the gut, mammary, and milk microbiomes in inbred mouse strains with known genetic and phenotypic diversity in mammary development and lactation, 2) Determine the ability of FMT derived from inbred female mice that are genetically distinct and phenotypically divergent in mammary ductal development and lactation to influence these traits in gnotobiotic C57BL/6J female mice. The completion of these studies will provide an unprecedented test of the capacity for the intestinal microbiome directly impact reproductive maturation, mammary development and lactation. These findings will also pave the way for in-depth work to understand the physiological and molecular mechanisms through which the microbiome shapes normal mammary biology. Gaining this understanding will also contribute to understanding breast diseases.